1. Field of the Invention
The present invention relates to a printed circuit board and a method of manufacturing the same. More particularly, the present invention relates to a printed circuit board, on which small electronic parts such as chips are mounted, and a method of manufacturing the same.
2. Description of the Related Art
Small electronic parts, such as semiconductor chips, are generally mounted on a printed circuit board. The printed circuit boards can be divided into flexible substrates and rigid substrates.
The rigid substrate typically consists of a raw material layer, an electro deposit copper (Cu) layer, and photo solder resist (PSR) ink. The electrolysis copper foil layer is formed on at least one surface of the raw material layer using a circuit lithography process, and the electrolysis copper foil layer is insulated by the PSR ink. In this case, the raw material layer on which the electrolysis copper foil layer is etched is called a core.
Materials used for forming the core of the rigid substrate, such as bismaleimide triazine (BT) or FR-4, include a prepreg and a resin material disposed around the prepreg. In this case, the prepreg denotes a composite of a glass fiber and a resin.
On the other hand, the flexible substrate is typically formed by a reel-to-reel process. A strip-cutting process for cutting the flexible substrate into strips is performed at the end of all manufacturing processes for forming the flexible substrate.
The rigid substrate is conventionally manufactured in strip units since it is less flexible than the flexible substrate. However, the strip process requires higher manufacturing costs than the reel-to-reel process. Particularly, for mass production, the strip unit process requires a higher cost than the reel-to-reel process by 20-60%.
Recently, as the size of semiconductor packages reduces according to the recent trend in light-weight, slim, short, and small products, the thickness of the substrate has become thinner. Accordingly, as the rigid substrate is thinner, the application of the reel-to-reel process is possible. Therefore, to reduce the manufacturing cost of the rigid substrate, the rigid substrate is manufactured by the reel-to-reel process.
However, when the rigid substrate is manufactured by the reel-to-reel process, bending of the rigid substrate and its core occurs. The main cause of the bending is the difference in the coefficient of thermal expansion between the PSR ink and the core. That is, the coefficient of thermal expansion of the PSR ink is approximately 5 to 6 times greater than that of the core. When a shear force is generated in the PSR ink due to thermal expansion of the PSR ink, bending from the PSR ink toward the core is generated. In this case, the bending of the PSR ink cannot be stopped by the core since the core is thin. The bending of the PSR ink generates cracks between the PSR ink and the core and within the core between the prepreg and the resin material.
Meanwhile, when a chip has to be mounted on the printed circuit board, a device hole can be formed in the printed circuit board using a puncher. Printed circuit boards disclosed in U.S. Pat. Nos. 6,080,684 and 6,136,733 are manufactured by inserting and sintering a glass fiber matrix into a resin material, forming a conductive film on the surface of the hardened resin, forming a circuit pattern by an etching process, and forming device holes by punching the resin material where the chip will be mounted.
When the substrate is a dual-sided substrate in which copper foil layers are formed on both sides of the core, a via hole for connecting the copper foil layers formed on the both sides of the core is formed.
Conventionally, to form the via holes, after sheet shaped cores are stacked in a multiple layer, the holes are mechanically formed using a drill while pressing the sheet shaped cores. In this case, to precisely form the via holes, holes having a certain size are formed in advance using a laser beam in the locations where the via holes are to be formed. Afterward, the via holes or the device holes are formed using a drill. That is, since a laser process and a drilling process must be performed, the manufacturing costs of the printed circuit board increase.
To reduce the manufacturing costs for making the via holes, a punching process can be performed. However, due to the glass fiber included in the printed circuit board, cracks can be generated at punched edges when the printed circuit board is pressed by the punch for punching the device holes or the via holes. The cracks are generated due to the difference in tensile strength between the glass fiber and the resin material. The resin material is readily punched by the pressure of the puncher, but the glass fiber is cut after the glass fiber is bent to some extent. The cracks are generated at boundaries between the conductive film and the resin material, between the glass fiber and the resin material, and between the glass fibers due to the difference in tensile strengths of the materials. The cracks not only degrade the appearance of the products but also adversely affect the fine circuit pattern formed on the conductive film.
Also, when the glass fiber is extended by the puncher while the holes are formed, the extended glass fiber presses the peripheral resin material, thereby deforming the resin material. As a result, white burrs, i.e., tiny cracks generated around the device holes or the via holes, may extend to a surface of the printed circuit board. The fine white burrs of the glass fiber remaining on inner surfaces of the device holes from time to time may cause troubles in a subsequent process.
Also, after the chips are bonded, the bonded portions are molded to protect the bonding portions using an epoxy molding compound (EMC). However, the EMC sometimes may not completely fill the cracks, thereby generating voids and further reducing the reliability of products.